Gas pyrolysis



Patented Mar. 2, 1937 GAS PYR P-ATENT OFFICE OLYSIS Wright W. Gary,Great Neck, N. Y., assixnor to Gasoline Products Company, Inc., Newark,N. J., a corporation of Delaware No Drawing. Application November 30,1934, Serial No. 755,413 '6 Claims. (01. 196-10) This invention relatesto the pyrolysis of hydrocarbon gases and particularly to a method ofeffecting such pyrolysis in which the gases to be pyrolyzed or crackedare subjected to high conversion temperatures while passing throughmetal tubes located in a heating furnace or other metal apparatusmaintained at the desired temperatures.

It has been known for some time that gaseous hydrocarbons may bepyrolyzed at elevated temperatures to effect a conversion thereof toproducts of greater value or to intermediate products which maysubsequently be converted to products of greater value. Thus, gaseousparaifins such as the fixed gases produced in the pyrolyticdecomposition of petroleum oils and comprising such individual parafiinsas ethane, propane and butane may be converted to gaseous olefins byheat treatment at temperatures of from 1200 to 2 1500 F., and thegaseous olefins thus produced may subsequently be polymerized to formvaluable low-boiling hydrocarbon liquids suitable for use as motor fuel.Or, gases containing parafiins or olefins or a mixture of both may besubjected to such temperatures or even higher temperatures to efiect thedirect pyrolysis of such hydrocarbon gases to low-boiling normallyliquid hydrocarbons.

In such processes, which present invention I prefer as gas pyrolysisnarily conducted metal tubes of su refractory under 11; is ordinarilystructed of met from oxidation for the purpose of the to designategenerally processes, the pyrolysis is ordiwhile gases are passingthrough ch character as to be sufiiciently the temperatures employed,and preferred to employ tubes conal which is as free as possible whichwould result in the formation of oxide film having an unfavorablecatalytic action upon the desired pyrolysis reactions. During the courseof the process, however, I have observed that the effectiveness of thetubes decreases. A considerable part of this loss in efiiciency is dueto the fact that more or less deposition of carbon takes place in thetubes, this deposition of carbon interfering mechanically with the useof the tubes and causing a tendency toward local overheating andpossibly also interfering with any beneficient catalytic effect the tubemetal may have upon the reactions taking place.

The formation of a film of 0 surface of the tubes, howeve duce theefficiency of the latter. Whether this is due to the direct eifect ofthe oxide film itself or whether the efiect is more complicated I amxide on the interior r, also tends to repyrolysis reactions wh not ableto say, but nevertheless the presence of oxide film does reduce thebeneficial catalytic effect of the tubes. While I do not Wish to limitmy invention by any theory, I am inclined to believe that'such filmexerts a catalytic action tend- 5 ing to reduce the rate of the desiredpyrolysis reactions.

I have further observed that this decrease in efficiency is especiallyapparent after the tubes have been burned out for the purpose of re- 9moving carbon. Thus, in the courseof the gaspyrolysis process, theformation of carbon may make it desirable to occasionally interrupt theprocess and to pass air or other oxygen-containing gas through the tubesto remove carbon therefrom while the tubes are maintained at asufficiently high temperature to promote combustion of the carbondeposits within the tubes. The passage of oxygen-containing gas throughthe tubes at high temperatures causes the forma- 20 tion of aconsiderable amountof oxide film on the interior surfaces of the tubesand tubes which have been thus treated are found to be greatly impairedin respect of their effect upon the gasen the gas pyrolysis opera- 25tion is resumed.

It is therefore an object of my invention to provide a method formaintaining the activity and efliciency of metal tubes employed for gaspyrolysis, whereby the pyrolysis of the gases may 30 be conducted over aconsiderable period of time without material decrease in efiiciency, andmy invention has for further objects such additional operativeadvantages and improvements as may hereinafter be found to obtain.

According to a preferred embodiment of my process, the pyrolysis of thehydrocarbon gases is conducted at the desired elevated temperatures,forexample 1200 F. or higher, while the gases are passed through metaltubes. .While the tubes employed may'be made of various metal alloysadapted to withstand the temperatures employed, it is ordinarilypreferred to employ a metal tube free from unfavorable catalyticproperties with respect to the gas-pyrolysis reactions, and inparticular alloys which have been found highly suitable arehigh-chromium steel alloys and alloys of steel, nickel and chromium, forexample a lowcarbon steel alloy containing approximately 18% of chromiumand 8% nickel. These tubes, when yed, possess no detrimental cata- I ngas pyrolysis, but after continued cially after being burned out for theemoving carbon therefrom or after currence which may result in a foruse,and espe purpose of r any other 00 the tubes, suffer considerable lossin efficiency as has been pointed out hereinabove.

It may here be observed that while my invention is especially adapted tobe employed as an adjunct to the use of such high-chromium alloy tubesfor gas pyrolysis and more especially the use of tubes made from thechromium-nickel alloy specifically mentioned hereinabove, my inventionis not in its broadest aspects limited in application to tubes of thisparticular character but may advantageously be employed in conjunctionwith the use of apparatus constructed of any metal I which is subject tothe formation of oxide films with resultant unfavorable effect .uponcatalytic efiiciency insofar as gas pyrolysis is concerned.

When, during the course of the gas-pyrolysis operation,

after the operation has been tubes have been allowed to cool to thedesired extent, a solution of the char acter indicated may be introducedinto the interior of the tubes, the tubes being maintained in contactwith the solution for a period of, for example, from one to two ho or atany rate until the oxide film has been removed.

This may readily be accomplished by merely pumping the acid solutionthrough the tubes in the furnace or other heating element employed.After the tubes have been pickled to the desired extent, the interiorsurfaces are then washed with water to remove the acid and then afterremoval of water and drying if necessary they may be utilized once morefor gas pyrolysis.

I have found, however, that it is extremely beneficial to subject theinterior surface of the tubes, before they are again placed on stream inthe gas-pyrolysis operation, to the action of a purifying medium such,for example, as concentrated nitric acid. The nitric acid may be passedthrough the tubes in the manner heretofore described with respect to thepickling solution. This procedure has been found to be of considerable'advantage in reducing the subsequent tendency of the tubes to form oxidefilm and greatly reduces or eliminates the precautions which mightotherwise be necessary to prevent the formation of slight oxide filmsupon mere contact with the atmosphere.

While the pickling in the tubes may be carried out as indicatedhereinabove, whenever the tubes have accumulated sufficient oxide filmto reduce their efliciency under ordinary circumstances, I have foundthat it is desirable to accomplish this only after the tubes'have beensubjected to the action of oxygen-containing gas for the removal ofcarbon therefrom. Under these circumstances, the gas-pyrolysis operationis interrupted at such a time as may. seem expedient for the removal ofcarbon and the carbon is first removed by passing air or otheroxygen-containing gas through the tubes at an elevated temperature, forexample at a temperature around 1000" F. After the removal of carbon hasbeen effected, the tubes may then be cooled to the desired extent andpickled in it becomes evident that the formation. of oxide film hastaken place to such extent as to .a,o7a,so1 I mation of oxide film onthe interior surfaces of accordance with my invention as set forthhereinabove before being employed again for further gas pyrolysis.

,-. It may be observed that the process constituting theory but on thecontrary upon the actual advantageous results which have been secured byits use.

Moreover, it will be apparent to those skilled in the art that, while Ihave described my invention with respect to a gas pyrolysis operation inwhicha conversion of hydrocarbon gases is effected while the gases arebeing passed through an elongated coil or tube or series of tubes, myinvention may be employed with'good results in conjunction with the useof other types of metal apparatus such as enlarged reaction vessels and.the like, and in conjunction with various types of processes for thepyrolysis of hydrocarbons. The advantages of the process are, however,more apparent in conjunction with the pyrolysis of hydrocarbon gases toolefins or to normally liquid products as aforesaid.

Furthermore, while I have given hereinabove various specific examples ofprocedures and pickling materials, it will be obvious to those skilledin the art that my invention is not in its broadest aspects limited tothe details of the illustrative examples given hereinabove, but mayvariously be practiced and employed within the scope of the claimshereinafter made.

I claim:

1. In a process for eifecting the pyrogenic conversion of gaseoushydrocarbons in metallic apparatus, the efiectiveness of which becomesreduced during the course of the operation, the steps which compriseinterrupting the gas-conversion operation when the effectiveness of theapparatus becomes thus reduced, passing an oxygen-containing gas throughthe apparatus to remove carbon, cooling the apparatus, pickling theinterior surfaces of the apparatus and reemploying the apparatus for gasconversion.

2. In a process for effecting the pyrogenic conversion of gaseoushydrocarbons in metallic apparatus, the effectiveness of which becomesreduced during the course of the operation, the steps which compriseinterrupting the gas-conversion operation when the effectiveness of theapparatus becomes thus reduced, -cooling the apparatus, pickling theinterior surfaces of the apparatus, treating said surfaces with apurifying agent, and re-employing the apparatus for gas conversion.

3. In a process for effecting the pyrogenic conversion of gaseoushydrocarbons in metallic apparatus, the effectiveness of which becomesreduced during the course of the operation, the steps which compriseinterrupting the gas-comversion operation when the effectiveness of theapparatus becomes thus reduced, cooling the apparatus, pickling theinterior surfaces of the apparatus with an acid solution to remove oxidefilm, removing said acid and re-employing the apparatus for gasconversion.

employing the apparatus for gas conversion.

5. In a process for effecting the pyrogenic conversion of gaseoushydrocarbons in highchromium alloy tubes, the effectiveness of whichbecomes reduced during the course of the operation, the steps whichcomprise interrupting the gas-conversion operation when theeifectiveness of said tubes becomes thus reduced, passing anoxygen-containing gas through the tubes to remove carbon, cooling thetubes, pickling the interior surfaces of the tubes with a solution ofsulphuric acid and sodium chloride, treating the tubes with nitric acidto purify the interior surfaces thereof, removing said acid andre-employing the apparatus for gas conversion.

6. The method of maintaining the catalytic efiiciency of metallicchromium alloy tubes employed for the pyrogenic conversion ofhydrocarbon gases, which comprises passing air through said tubes athigh temperature to remove carbon therefrom, then passing an acidpickling solution through the tubes to romove oxide film therefrom, andremoving said acid 15 solution.

WRIGHT W. GARY.

